A gyroscope is a sensor that measures the rate of rotation of an object. The concept of a vibrating MEMS (Micro-Electro-Mechanical System) gyroscope is to generate momentum of a proof-mass to induce and detect the Coriolis force. A Coriolis force is applied to the proof-mass in motion when an angular rate is applied. The Coriolis force Fc is the product of the proof-mass m, the input rate Ω and the mass velocity v. The direction of the Coriolis force is perpendicular to the motion of the proof-mass.
The basic architecture of a vibratory gyroscope is comprised of a drive-mode oscillator circuit that generates and maintains a constant linear momentum of the proof-mass, and a sense mode circuit that measures the sinusoidal Coriolis force induced due to the combination of the drive oscillation and any angular rate input. The majority of vibratory gyroscopes utilize a vibratory proof-mass suspended by springs above a substrate. The objective being to form a vibratory drive oscillator coupled to an orthogonal sense system detecting the Coriolis force.
Since the Coriolis Effect is based on conservation of momentum, the drive-mode oscillator circuit is implemented to provoke the oscillation of the proof-mass which is the source of this momentum.
FIG. 1 illustrates a simplified block diagram of an example of a MEMS gyroscope implementation 100. In such a MEMS gyroscope implementation 100, a drive-mode oscillator circuit 110 vibrates the proof-mass 120, causing the proof-mass 120 to oscillate. When an angular rate is applied to the proof-mass 120, the motion of the proof-mass 120 is deflected in a direction perpendicular to the direction of oscillation of the proof-mass (sense mode). The amount of deflection may then be measured via sense electrodes and used to determine the angular rate that was applied to the proof-mass.
Due to the mechanical properties of such MEMS devices, the drive-mode oscillation circuit 110 is required to operate at a resonance frequency of the proof-mass 120. In a typical MEMS gyroscope implementation, the drive-mode oscillator circuit 110 is based on a self-oscillating loop principle in which the proof-mass motion is detected, phase-shifted, amplified and used as an electrical stimulus to drive the proof-mass oscillation.
FIG. 2 illustrates a simplified block diagram of an example of such a conventional drive-mode oscillator circuit 110. The drive-mode oscillator circuit 110 in the illustrated example comprises a capacitance to voltage (C2V) circuit 210 arranged to convert a capacitance change of differential MEMS drive measurement units (DMUs) 200, 205 caused by the displacement of the proof-mass to a differential voltage measurement signal 215. An integrator 220 receives the voltage measurement signal and phase shifts it by, for example, 90° to compensate for the phase lag of the system. A voltage gain amplifier (VGA) 230 receives the phase shifted voltage signal 225 and outputs a proof-mass drive signal 235 to differential drive actuation units (DAUs) 240, 245, which vibrate the proof-mass 120 accordingly. An automatic gain control (AGC) circuit 250 provides a control signal 255 to the VGA 230 to control the amplitude of the proof-mass drive signal 235 output thereby. Conventionally, such an AGC circuit 250 is implemented using complex analogue circuitry for providing the necessary gain correction, which tends to require high current consumption and a large die-size, and is prone to temperature and process variations.